Depression is a major public health problem. Even slight elevations in depressive symptoms are associated with significant deficits in daily functioning. Only 50% of Americans with depression receive adequate treatment, and one-third of those receiving treatment do not benefit. There is now evidence that internet-based depression interventions can effectively reach large numbers of people at low cost, but they commonly have problems with high attrition and low adherence. We propose an intervention to reduce depressive symptoms by adapting our existing internet- based positive affect skills intervention for individuals with depression. We will develop and test innovative features for increasing adherence including exercises for use on mobile devices in real-life situations; very brief contact with a study facilitator; peer support via a private, moderated internet forum; and optional goals and challenges that provide users with reinforcement and feedback. The materials and delivery method will undergo multiple cycles of user testing and comparative evaluation, followed by a randomized test comparing several different versions of the intervention. We aim to 1) Maximize acceptability and relevance of intervention content for people with elevated depressive symptoms or clinical depression; (2) Develop methods for boosting retention and adherence, and (3) collect preliminary data regarding proximal mediators of efficacy of the program for reducing levels of depressive symptoms. The proposed study will extend the literature and inform clinical practice by gathering evidence regarding efficacy of positive psychological interventions; optimizing an online intervention for depression that has the potential for widespread reach and dissemination; and maximizing retention and adherence, two key components of successful psychological interventions.